1. Field of the Invention
The present invention relates to a liquid crystal display panel and a driving method for a liquid crystal display panel, especially relates to a method to drive the potential of a control electrode higher than the potential of a pixel electrode.
2. Description of the Related Art
With the wide applications of liquid crystal display (LCD) panels, users have more and more demands about the quality of the LCD panel, such as high brightness, high contrast, high resolution, high color saturation and fast response time. Especially as the panel size increases, the LCD panels have generally been applied to household flat displays, such as liquid crystal (LC) TV sets, which have become an important application of the LCD panels. Most of the general, traditional LCD panels have narrow view angles so the normal images displayed by them only can be viewed directly in front of the display area. If we watch the display area from an oblique view angle, color distortion occurs in what we watch, and even gray inversion occurs. That is, what appears black is actually white and what appears white is actually black. Therefore, how to widen the view angle is an important subject for the LCD manufacturers.
Among various methods for widening the view angle, an LC Vertical Alignment (VA) technique is still one of the most popular techniques in the current LCD market. However, because liquid crystal molecules are aligned in the same direction (mono-domain vertical alignment), we also cannot see a normal image from the view angle perpendicular to or symmetric to the direction. No matter when the liquid crystal molecules are realigned in a different direction after the electrical field existing therein changes, the view angle is also limited to the parallel direction of the liquid crystal molecules. Therefore, a multi-domain VA technique was put forth to improve the drawback of the prior art, hence the quality of various view angles is assured. Japanese Fujitsu Corporation once tried to form ridges or bumps on the color filter, and use the oblique boundary generated by bumps to control the alignment of the tilt direction of liquid crystal molecules automatically align tilt direction according to where region their belong to. But because the existence of the bumps results in that the precise alignment between a color filter and an active matrix substrate is necessary, and an additional over coating is necessarily formed on the color filter, the yield of this LCD panel becomes worse and the cost thereof increases.
FIG. 1 is a cross-sectional diagram of a conventional LCD display panel with a bias-bending vertical alignment (BBVA) type. The LCD panel 10 comprises a color filter 11, a liquid crystal layer 12 and an active matrix substrate 13. The color filter 11 and active matrix substrate 13 have a transparent substrate 111 and 131 respectively. A main electric field exists between the common electrode 112 formed on the color filter 11 and the pixel electrode 134 formed on the active matrix substrate 13, and a pair of symmetrically oblique electric fields exists between a control electrode 133 and the pixel electrode 134 together formed on the active matrix substrate 13 to make liquid crystal molecules 121 have oblique positions. There is another insulation layer 132 interposed between the control electrode 133 and the pixel electrode 134.
But when VCE<Vcom<VP is satisfied, a declination line is brought into existence in the center of an area B, wherein VCE, Vcom and Vp represent the potentials of the control electrode, common electrode and pixel electrode respectively. The existence of the declination line result in that the liquid crystal layer 12 has a lower transmission ratio, a longer response time and an unstable status. In order to avoid the occurrence of these negative phenomena, it is expect that the following criteria should be satisfied during polarity inversion:
Criterion 1: If the current pixel is a positive frame, then VCE>Vp>Vcom; and
Criterion 2: If the current pixel is a negative frame, then VCE<Vp<Vcom.
FIG. 2 is an equivalent circuit diagram of a pixel proposed by Korean Samsung Electronics Cooperation. The circuit of pixel 20 can satisfy aforesaid criteria to eliminate declination lines. One electrode of the first thin film transistor T1″ is connected to a data line 262, and the gate electrode of it is driven by the scanning line 252. When the first thin film transistor T1″ is turned on, the data signal of the data line 262 is written into a pixel electrode 24. One electrode of the second thin film transistor T2″ is connected to a data line 261, and the gate electrode of it is driven by a scanning line 251. When the second thin film transistor T2″ is turned on, the data signal of the data line 261 is written into a control electrode 23. One electrode of the third thin film transistor T3″ is connected to the data line 262, and the gate electrode of it is driven by the scanning line 251. When the third thin film transistor T3″ is turned on, the data signal of the data 262 is written into the pixel electrode 24.
In the pixel 20, a liquid crystal capacitor C1″ exists between the pixel electrode 24 and common electrode 27, a Bias-Bending capacitor C2″ exists between the control electrode 23 and pixel electrode 24, and a capacitor C3″ exists between the control electrode 23 and the common electrode 27. Therefore, we obtain the following formula:
            V      CE        =                                        C            2            ″                                              C              2              ″                        +                          C              3              ″                                      ⁢                  (                                    V                              d                ⁢                                                                  ⁢                1                                      +                          V                              d                ⁢                                                                  ⁢                3                                              )                    +              V                  d          ⁢                                          ⁢          2                      ,wherein Vd1, Vd2 and Vd3 respectively represents the potentials of pixels, dividedly placed on coordinate (n,m), coordinate (n−1,m−1) and coordinate (n−1,m), to which the data signals are respectively applied. Meantime, we obtain an equation VCE−VP=Vd2+Vd3 to satisfy Criteria 1 and 2. However, because each of the pixels 20 includes three thin film transistors, only if one of the thin film transistors is damaged, the pixel is considered to be malfunctioning. Therefore, the manufacture yield of this LCD cannot meet an acceptable standard currently. On the other hand, the number of the thin film transistors connected to a same scanning line is too much so as to result in a severe RC delay on the scan signal. The foresaid problems have to be further resolved.